A novel sulphonated phenolic named celtisanin, elucidated as 3-[8'-hydroxy-prop-6'-ene-6'-yl]-5-hydroxymethyl-4-methoxy-2-[penta-1',4'-diene-1'-yl-5'-sulphonic acid]-5,6-dihydrobenzofuran, has been isolated from the fruits of Celtis australis (Ulmaceae), together with three known compounds: apigenin, quercetin and its glucoside. Their structures were characterised by means of chemical and spectral methods, including advanced 2-D nuclear magnetic resonance (NMR) studies.